January 02, 2008

The difference between SOA and 3-tier

Jeff Schneider recently started a thread on the Yahoo! service-orientated-architecture discussion list asking for a comparison between SOA and 3-tier. Here's my response:

3-tier or n-tier application architecture is just that -- an *application* architecture. It's focus is on building monolithic applications that can be distributed across multiple processors to increase performance and scalability. The tiers represent different aspects of the application: presentation, business logic, and data access. But the fundamental architecture doesn't talk about services, and it has never stressed reusability -- except at the DBMS layer.

SOA and n-tier are somewhat orthogonal -- you can build the various application tiers from services. Folks that built n-tier systems with CORBA or similar service-oriented technology were more likely to use services than most others. But consider the predominant n-tier development environment in use today -- Java EE:

presentation implemented with JSP/servlets

business logic implemented with POJOs or EJBs

data access logic implemented with JPA/Hibernate

I don't see much that looks like SOA in this model.

SOA operates at a significantly different level from n-tier. SOA is focused on reducing the amount of redundancy in the corporate application portfolio. I've said it before: SOA is an enterprise architectural style rather than an application architectural style. SOA doesn't focus on individual application architecture. It focuses on encapsulating discrete functionality into services.

SOA and n-tier architectures have one thing in common -- they focus on separation of concerns. But SOA takes SoC to a new level. I think of it as refactoring application systems. (similar to refactoring code,but on an enterprise application architectural level).

As industry experience with SOA matures, I expect we'll start to develop new application architectures that more effectively exploit a development model based on services. SCA is a start, although stillway too immature for prime time. I expect to see some synergy develop among RIA, widgets and gadgets, mashups, composite application development, and services.

I also expect that the concept of "application" is likely to go away. Why is it that we as systems users have to be constrained by the limits of this artificial boundary called an application? Why do I have to shift my focus among multiple applications? Why do I have to copy data from one application to another? Why do I have to launch this stupid browser or that stupid application to get my work done? Why isn't everything just accessible to me from my desktop (via widgets and gadgets) or from within my preferred operating context (e.g., email)? Why can't I just link to whatever I need? (think REST)

Think outside the box!

Chris Haddad talked a little about the need to abandon traditional n-tier concepts and the MVC design patterns in his blog post, MVC Matrix and the Red Pill.We'll be talking a lot more about this next generation application platform this year. Stay tuned.